ferous rocks. Lastly, if we consider the agencies which have been at work in 

 producing the different rocks, we may distinguish them into three groups, viz., 

 the Igneous, Metamorphic, and Aqueous formations. The first have been pro- 

 duced by the fusion of mineral matters by the action of heat ; the second, by the 

 action of heat in modifying previously deposited rocks; and the last have for the 

 most part been deposited in strata at the bottom of seas, rivers, and lakes. The 

 aqueous rocks have been divided into three great series, chiefly in reference to 

 their organic contents, viz., the Palaeozoic series, or Primary; the Mesozoic, or 

 Secondary; and the Cainozoic, or Tertiary. These several series, with the groups 

 they include, will be found stated at length on the Table of Geological Strata, 

 forming one of the plates of the Atlas. The Igneous, Metamorphic, and Fossili- 

 ferous rocks also, are described upon the several Diagrams illustrating them. 



Present Aspect Of the Globe. The outlines of land and sea throughout 

 the globe depend principally on the disposition and groups of mountain chains, 

 which in every instance yet known, are certainly shown to have been raised by 

 mechanical agency, generally the result of igneous action. Frequently, how- 

 ever, this dependence of the form of the existing land upon the ranges of 

 mountains is disguised by the extent of comparatively plain country which 

 separates the mountains from the sea. In such cases, it is necessary to admit 

 that the general level of the sea has subsided, or that large tracts of land have 

 been raised gradually, or by successive movements around the mountains, which 

 in earlier times may have been uplifted by more violent causes. 



The interior features of every country in like manner depend upon recognized 

 geological agencies. The unequal elevation of mountain ranges above the sea is 

 a phenomenon wnich will be found of great importance in geological theory. It 

 appears to be true, at least in Europe, that the most elevated chains of mountains 

 are those whose elevation was not completed until the tertiary or later epochs. 

 Raised in this manner by violent or gradual movements out of the sea, the dry 

 land has since been subjected to waste by atmospheric action. The formation of 

 valleys is due to the various effects of atmospheric agency; the action of running 

 waters; the subsidence of the crust of the earth; dislocations on the line of 

 the valley; or by the overwhelming force of a general flood. The forms 

 of hills, like the depth and direction of valleys, are in part dependent on the 

 presence of strata of unequal resisting power. 



The land visible on the surface of the globe is not all of the same antiquity; 

 some regions must have been covered with trees, and traversed by animals, before 

 the substance of others was laid on the bed of the sea. Since life was developed 

 on the globe, there appears never to have been any considerable period during 

 which the land or sea was wholly deprived of organic beings ; but as the condition 

 of the globe changed, the forms of life were altered, old races perished, new 

 creations were awakened, the sum of animal and vegetable existence was con- 

 tinually augmented, and the variety of their forms and habits continually 

 multiplied, until man was added to the wonders of creation. 



Economic Geology. As geology advances, its application to productive 

 industry becomes more and more valuable. The great aid aflorded by this 

 science to coal mining has been shown, in indicating where coal may or may not 

 be reasonably looked for, according to the nature of the adjacent strata. Of the 

 situation of metallic treasures, enough is known to show that the occurrence of 

 mineral veins is a circumstance depending on conditions which are more or less 

 ascertainable. In planning the lines of railways, canals, &c., the engineer will 

 often be benefited by the records of geological surveys. The careful researches 

 preparatory to the selection of stone for the new Houses of Parliament, afford an 

 example of the way in which geology may be brought to bear on the constructive 

 arts, as indicating the position, character, and extent of the different marbles, 

 limestones, clays, &c. To the agriculturalist, geology has rendered some services, 

 and probably may in future be appealed to for further aid. Geology is the basis 

 of all sound knowledge for ascertaining the position of springs and the subterra- 

 nean distribution of water. The rain which falls upon all soils and rocks 

 indifferently, runs off the clays, but sinks into the limestones, sandstones, and 

 other rocks, whose open joints act like so many hidden reservoirs Hence, a 

 knowledge of the subsequent course of these waters is of infinite importance to 

 the subject of drainage, the construction of wells, and to the supply of water to 



towns. 



(In part abridged from an able article in the National Cyclopaedia.) 



